CSS

font shorthand

/*Syntax*/
font: font-style font-variant font-weight font-size/line-height font-family;

/*In Use*/
font: italic small-caps normal 13px/150% Arial, Helvetica, sans-serif;

You need to supply at least 
1. font-size 
2. font-family 

body {
  font: italic 20px Serif; /* works */
  font: 20px; /* fails */
  font: 18em Fantasy; /* works */
  font: bold small-caps; /* fails */
}

font shorthand

/*Syntax*/
font: font-style/font-variant/font-weight/font-size/line-height/font-family;
/*Example*/
font: italic small-caps bold 18px Arial, Helvetica, sans-serif;
/*IMPORTANT NOT
1- You need to supply at least font-size and font-family. 
2- You need to respect the values order shown in the syntax 
*/

font shorthad

font: {font-style} {font-variant} {font-weight} {font-size/line-height} {font-family};